In a typical hydraulic pump-motor, an electric motor is provided to drive a hydraulic pump and increase the pressure of the hydraulic fluid processed through the pump. By varying the hydraulic load, the resulting output pressure of the hydraulic fluid is correspondingly varied and a broad range of pressures is obtainable.
However, the operation of the motor and pump results in considerable vibration of the pump-motor unit, which is communicated to the end shield of the motor and ultimately to those in the vicinity of the pump-motor in the form of noise. The shape and position of conventional motor end shields, as well as the material from which the end shields are typically manufactured, have contributed to a drum-like effect in that the end shield reverberates during operation and thereby produces considerable acoustic output. This problem is accentuated when the pump is of a reciprocating type in that with each movement of the piston a pulse is generated, and this pulsating output is transmitted to the end shield of the motor. Since such pump-motors are commonly used in commercial applications, e.g., dental chairs, cosmetology chairs, and hospital beds, this output noise has been the source of substantial annoyance and dissatisfaction.
The prior art has typically employed a standard die cast aluminum end shield which provides little, if any noise attenuation. Some improvement has been attained by making the end shield of cast iron as opposed to aluminum. Given the natural frequency of these metals and the frequency with which pump-motors vibrate, as well as the planar shape of typical end shields, these end shields have proven to have inadequate sound deadening characteristics and, in fact, act much like a drum as the pump-motor operates.
In an effort to improve noise reduction, the standard die cast aluminum end shields of typical pump-motors have been replaced by specially shaped cast iron, plastic, or other material end shields. Given the natural frequency and sound deadening effects of such shapes, this design has provided improved sound reduction, but even greater reduction in acoustic output is desirable.
Moreover, typical end shields are secured to the motor via the same bolts which hold the stator of the motor in place. Therefore the end shield must be made from a material with sufficient strength to provide a good clamping surface for the stator screws. However, materials with good sound attenuation qualities often do not have sufficient strength to provide a good clamping surface. In addition, since the same screws which hold the stator in place also hold the end shield in place, the prior art has had difficulty in accurately measuring the air gap between the rotor and stator, after the end shield has been fastened to the motor.